Recently, as semiconductor elements become highly integrated and the density of a package becomes high, arts to provide the semiconductor elements to have microscopic line width, are required.
Accordingly, methods for forming photo lithography masks having microscopic width are actively under research and development.
In general, photo lithography is an art for forming an image pattern on a surface of photo resist coated on a semiconductor substrate by passing rays like ultra-violet rays through a photo mask.
Such a photo mask consists of opaque pattern and transparent patterns, through patterns which rays like ultra-violet rays are passed selectively.
However, in such a photo lithography, diffraction phenomena develops as the density of the pattern increases, affecting the resolution.
As a method for solving such a problem, there is a method of photo lithography utilizing phase shifting mask.
A phase shifting mask is a method using combination of transparent areas phase shifted at 180 degrees against normal transparent areas, which develops an offset interference between transparent areas in opaque areas, preventing diffraction of lights.
And this phase shifting mask lithography technology can sharply demodulates the intensity of the light passing through the mask making the image of the pattern close to the image of the mask.
And many kind of phase shifting masks are under development so that even projections of more complicate pattern can be possible.
In the phase shifting masks, there are Levenson type and rim type.
In Levenson type, a transparent film shifting phase of light is formed on one part of transparent parts adjacent to other part. In rim type, a phase shifting layer having a fixed width is formed self-aligned around the periphery of a mask opening.
A foregoing rim type phase shifting mask is to be explained hereinafter, referring to the attached drawings.
Shown in FIGS. 1(a) to 1 (d) are sections explaining conventional method for fabrication of a mask, wherein, as shown in FIG. 1(a), an opaque layer 2 is deposited on a transparent substrate(glass or quartz), on which organic photo sensitive material 3 is coated.
Then, as shown in FIG. 1(b), a transparent area is defined by selective exposure and development, the opaque layer 2 of which is removed selectively to expose the transparent substrate. In this time, defects of the mask are repaired.
As shown in FIG. 1(c), the organic photo sensitive material 3 is removed with rotation coating method, on all over which phase shifting material of PMMA(Polymethyl Methacrylate) 4 is coated, which is exposed and developed using the opaque layer 2 as a mask to remove the PMMA 2 in the part having no opaque layer 2.
Then, by removing part of the opaque layer 2 with wet etching using the patterned PMMA 4 as a mask, the mask is completed. In the lithography mask structure fabricated as in the foregoing description, as shown in FIG. 1(d), an opaque layer 2 is formed on the transparent substrate 1 to block light selectively, on which opaque layer 2 phase shifting material of PMMA 4 is formed extending more into the transparent area from the edges of the opaque layer 2.
However, such a conventional lithography mask fabrication method has following problems.
First, control of thickness is difficult because the method of fabrication is carried out simply by forming phase shifting area of the PMMA 4 with rotation coating method.
Second, the flatness of phase shifting material of the PMMA 4 becomes bad due to the steps of the opaque layer.
Third, phase shifting material of the PMMA is susceptible to damage during handling of the mask.
Fourth, phase shifting material of PMMA has high possibility of particle infiltration during handling of the mask.
Fifth, phase shifting effect can be decreased due to development of under cut because the side walls of the opaque layer are formed with a wet etching.